Neutral carbon-based η1 ligands have previously been limited to carbon monoxide, isocyanides, and carbenes. Carbenes tend to bind more strongly to metal centers than do phosphorus-based ligands. Because of this strong carbene-metal bond, the use of carbene ligands in metal coordination complexes avoids the need for excess ligands in catalytic reactions related thereto. In addition, metal coordination complexes that include carbene ligands are often less sensitive to air and moisture and are resistant to oxidation (E. Peris, et al., C. R. Chimie 2003, 6, 33-37). The robustness of carbene-containing metal coordination complexes is also largely due to the presence of strong carbon-metal bonds. Based on the beneficial properties that result from strong carbon-metal bonds in carbene-containing metal coordination complexes, other types of carbon-based L ligands are desirable.
Metal coordination complexes that include carbene ligands have been known since the 1960's (K. Ofele, J. Organomet. Chem., 1968, 12, P42-P43; D. J. Cardin, et al., Chem. Rev. 1972, 72, 545-574). Despite being known since the 1960's, the number of catalytic applications that incorporate carbenes (J. C. Y. Lin, et al., Chem. Rev., 2009, 109, 3561-3598; P. L. Arnold, et al., Chem. Rev., 2009, 109, 3599-3611; S. Diez-Gonzalez, et al., Chem. Rev., 2009, 109, 3612-3676; M. Poyatos, et al., Chem. Rev., 2009, 109, 3677-3707; C. Samojlowicz, Chem. Rev., 2009, 109, 3708-3742; W. A. L. van Otterlo, et al., Chem. Rev., 2009, 109, 3743-3782; S. Monfette, et al., Chem. Rev. 2009, 109, 3783-3816; and B. Alcaide, Chem. Rev., 2009, 109, 3817-3858) has increased in large part because of the recent availability of carbenes which are stable enough to be bottled (A. Igau, et al., J. Am. Chem. Soc., 1988, 110, 6463-6466; A. Igau, et al., Angew. Chem., 1989, 101, 617-618; Angew. Chem. Int. Ed., 1989, 28, 621-622; A. J. Arduengo III, et al., J. Am. Chem. Soc., 1991, 113, 361-363; A. J. Arduengo III, et al., J. Am. Chem. Soc., 1995, 117, 11027-11028; D. Enders, Angew. Chem., 1995, 107, 1119-1122; Angew. Chem. Int. Ed., 1995, 34, 1021-1023; M. Melaimi, et al., Angew. Chem. Int. Ed., 2010, 49, 8810-8849; D. Tapu, et al., Chem. Rev. 2009, 109, 3385-3407; J. Vignolle, et al., Chem. Rev. 2009, 109, 3333-3384; F. E. Hahn, et al., Angew. Chem. 2008, 120, 3166-3216; Angew. Chem. Int. Ed., 2008, 47, 3122-3172; Y. Canac, et al., J. Organomet. Chem., 2004, 689, 3857-3865; D. Bourissou, et al., Chem. Rev. 2000, 100, 39-91). Carbenes, such as imidazol-2-ylidenes (U.S. Provisional Patent Application 61/393,841, filed Oct. 15, 2010) which are exemplified by compound 1 in FIG. 1, and 1,2,3-triazol-5-ylidenes (G. Guisado-Barrios, et al., Angew. Chem. Int. Ed., 2010, 49, 4759-4762), which are exemplified by compound 2 in FIG. 1, are recognized for their advantageous properties as ligands for metal coordination complexes and in catalytic applications (C. Grondal, et al., Nat. Chem., 2010, 2, 167-178; D. Enders, et al., Chem. Rev., 2007, 107, 5606-5655; V. Nair, et al., Chem. Soc. Rev., 2008, 37, 2691-2698; N. E. Kamber, et al., Chem. Rev., 2007, 107, 5813-5840).
What is needed in the art are compounds of, and methods for preparing, stable mesoionic carbenes, i.e. 1,2,3-triazol-5-ylidenes. Surprisingly, the present invention meets these as well as other needs.